WASHINGTON (Reuters) - The U.S. Navy deliberately avoided military drills or other actions that could have further inflamed tensions with Beijing during a patrol last week near islands China has built in the South China Sea, U.S. officials said.

Subi reef, located in the disputed Spratly Islands in the South China Sea, is shown in this handout Center for Strategic and International Studies (CSIS) Asia Maritime Transparency Initiative satellite image taken September 3, 2015 and released to Reuters October 27, 2015. REUTERS/CSIS Asia Maritime Transparency Initiative/DigitalGlobe/Handout via Reuters

“We wanted to assert our rights under international law, but not to the point where we were poking the Chinese in the eye, or where it would unnecessarily escalate the situation,” said a U.S. official, speaking on condition of anonymity.

The official said the destroyer USS Lassen turned off its fire control radars while transiting within 12 nautical miles of Subi Reef and avoided any military operations during that time, including helicopter launches or other drills.

Numerous experts said this cautious approach could in fact reinforce China’s claim to sovereignty over the artificial islands in the Spratly Islands archipelago.

But the U.S. official disputed that assertion.

“It was a freedom-of-navigation operation that was not meant to inflame the situation, which is why they did the transit the way they did,” the U.S. official said.

The Lassen’s commanding officer, Commander Robert C. Francis Jr, told reporters on Thursday that his ship went within six to seven nautical miles of the artificial island.

He said the radar was operating normally at the time for “situational awareness,” and acknowledged the U.S. Navy did not fly helicopters. He described it as both a freedom-of-navigation and a “transit” operation.

China reacted angrily to the patrol, which followed months of U.S. preparation, despite its lack of military drills.

But analysts said that if the Lassen failed to take such actions or even to loiter or collect intelligence within the zone, the operation would have resembled what is known as “innocent passage,” and could have reinforced rather than challenged China’s claim to a territorial limit around the reef.

“Innocent passage” occurs when one country’s ship quickly transits another’s territorial waters - and can only take place in waters belonging to another country.

“If the Lassen didn’t do anything but transit, then this Freedom of Navigation Operation didn’t actually assert what they had led us to believe it was supposed to: that Subi Reef doesn’t get a territorial sea,” said Greg Poling, director of the Asia Maritime Transparency Initiative at the Center for Strategic and International Studies think-tank.

INNOCENT PASSAGE?

Julian Ku, a professor of constitutional law at Hofstra University, wrote in the Lawfare blog that Washington had chosen the weakest type of freedom-of-navigation operation available, apparently at the bidding of the White House.

“(B)y limiting the USS Lassen’s transit to an ‘innocent passage,’ the U.S. is implicitly recognizing that China is entitled to a 12 nm (nautical mile) territorial sea around its artificial island on Subi Reef,” he said.

The White House declined comment on details of the operation. A senior administration official called it “consistent with the way we regularly conduct freedom-of-navigation operations globally.”

Underscoring the issue’s complexity, Pentagon officials have given conflicting descriptions over the last week of the Lassen’s maneuver.

A U.S. official speaking to Reuters last week described the patrols as an “innocent passage” operation, but later said that had been a mistake.

Pentagon spokesman Captain Jeff Davis insisted to reporters on Wednesday that the patrol was not an “innocent passage.” Pressed further on the issue on Thursday, he declined to explicitly restate that position or elaborate.

Writing on the website of The National Interest magazine, Bonnie Tyler, senior adviser for Asia at CSIS and Peter Dutton, director of the China Maritime Studies Institute at the U.S. Naval War College, offered an explanation for the mixed messages.

They concluded that the operation was a freedom-of-navigation movement, but carried out as innocent passage, as it also passed within 12 nautical miles of a China-claimed feature that was entitled to that territorial limit.

The first U.S. official argued that China - which described the U.S. patrol as “illegal” - was not seizing on the absence of military activities as a sign that Washington now accepted its sovereignty over the artificial islands.

“It didn’t change anything in the way it was received. What the Chinese took away was that we steamed through what they believe is their waters,” the official said.

In a joint paper, Adam Klein, a fellow at the Council on Foreign Relations, and Mira Rapp Hooper, a senior fellow in the Asia-Pacific Security Program at the Center for a New American Security, said the lack of clarity over the operation was a problem.

“If allowed to harden, the widespread belief that the Lassen conducted innocent passage would be extremely damaging; indeed, it could make the operation worse than having done nothing at all,” they wrote.

“The Pentagon needs to clarify what happened at Subi Reef - and, more importantly, what message it intended to send.”